There is conventionally known a fuel vapor treatment apparatus in which fuel vapor produced in a fuel tank is temporarily adsorbed in a canister. The fuel vapor is desorbed from the canister and is introduced and purged into an intake passage of an internal combustion engine. This type of fuel vapor treatment apparatus is disclosed, for example, in Japanese Patent Publication Nos. JP-A-5-18326 and JP-A-6-101534.
In such a fuel vapor treatment apparatus, an electric current is fed to a electromagnetic purge valve provided in the purge passage that is, for example, duty ratio-controlled, thereby adjusting a quantity of fuel vapor flowing through the purge valve. In addition, a flow quantity of purged fuel vapor and an injection quantity of fuel injected from a fuel injection valve are combined to realize a target air-fuel ratio in accordance with an engine operating condition.
However, as shown in FIG. 5, for the same duty ratio supplied to the purge valve, there can be variations in flow quantity of fuel vapor flowing in the purge valve due to variations in flow characteristic for each purge valve or variations in flow characteristic due to age. Accordingly, even if a flow quantity of the purge valve is controlled to obtain a target air-fuel ratio based upon the flow characteristic of the purge valve, the actual quantity of fuel vapor to be purged into the intake passage may differ from the target value. As a result, an actual air-fuel ratio in the internal combustion engine may deviate from the target air-fuel ratio.
Furthermore, where a fuel vapor treatment apparatus is applied to a low-pressure engine (i.e., where a negative pressure produced in the intake passage is reduced for less fuel consumption), the passage area of the purge valve is increased. However, increasing passage area of the purge valve causes increased variation of flow characteristic in each purge valve.
For eliminating such variation, manufacturing precision can be improved and/or a flow adjustment mechanism or the like can be added. However, these methods are limited. In a case of performing linear control of a purge valve, for example, where current value supplied to the purge valve and shift amount of the purge valve member are feedback-controlled, a shift sensor for detecting a shift amount of the valve member in the purge valve is needed, thus increasing costs.
Further, it is desirable to measure a flow quantity of fluid flowing in the purge valve to correct a flow characteristic of the purge valve. However, since the fluid flowing in the purge valve is a mixture of fuel vapor and air, even for the same duty ratio or current value, flow quantity of the fluid can vary with density of the fuel vapor or properties of the fuel. As a result, it is difficult to accurately correct the flow characteristic of the purge valve, and yet since a fuel vapor quantity out of a target value is purged until the flow characteristic is corrected, air-fuel ratio can deviate from the target value. Thus, there remains a need for a fuel vapor treatment apparatus that controls a quantity of fuel vapor to be purged regardless of variations in flow characteristic in each purge valve, with high accuracy, and inexpensively.